Self-adapting limiter

ABSTRACT

A limiter for limiting selected frequency components by generating Stokes waves in a stimulated Brillouin scattering medium. The generated Stokes waves create a seed that is provided to another stimulated Brillouin scattering medium. The seed selecting the undesired frequency components to be attenuated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 60/426,732, filed Nov. 15, 2002 and entitled“Self-Adapting Limiter” the disclosure of which is hereby incorporatedherein by reference.

This application is related to an international patent applicationentitled “Agile Spread Waveform Generator and Photonic Oscillator” byDaniel Yap and Keyvan Sayyah, filed on Nov. 15, 2002 under theprovisions of the Patent Cooperation Treaty (PCT), at the United StatesReceiving Office, bearing the serial number PCT/US02/36849 (AttorneyDocket No. 620376-1), which application designates the United States andother countries, the entire disclosure of which is hereby incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to Stimulated Brillouin Scattering (SBS).More particularly, the present invention relates to using the limitedbandwidth of SBS to selectively attenuate certain modulation sidebandsof a RF modulated lightwave waveform, while not attenuating othermodulation sidebands.

2. Background of the Invention

Many RF receivers such as those used for communication and radar systemsare located in complex and densely populated electromagnetic (EM)environments. The signal emitted from one antenna can interfere with thereception of another antenna. For example, commercial receivers locatednear high-power transmitters such as television or radio stations aresubject to substantial interference. The intensity of the interferingsignals can be many orders of magnitude higher than the desired signalsto be sensed by the receiving system, such as a radar system. The highpower in the interfering signal components can saturate the amplifiersin the receiver and thus distort desired signals. They also placegreater demands on the dynamic range required of digital receivers andtheir analog-to- digital convertors.

Present receivers address the problem of interference by usingfrequency-notch filters and multiple stages of frequency conversion toremove known interference. Also, multiple stages of automatic gaincontrol (AGC) and limiting are used to prevent saturation of theelectronics and to extend their linear range to higher input powerlevels. However, multiple stages of AGC and/or limiting reduce thesensitivity of the sensor. Actual systems might include more than 75 dBof gain reduction, distributed along the entire receive path, with anaccompanying degradation of the noise figure. Prior electronic limiterstypically are amplifiers whose gains become clamped once the intensityof the composite input signal reaches or exceeds a certain value. Thatclamping has no frequency selectivity and applies to all frequencycomponents of the input. Thus, desired frequency components are alsoadversely affected. AGC amplifiers likewise have no frequencyselectivity. What is needed is a limiting system and method forselectively attenuating certain frequency components while notattenuating other frequency components.

Stimulated Brillouin Scattering (SBS) has been used to selectivelyattenuate the optical carrier of an amplitude-modulated RF lightwavesignal, see U.S. Statutory Invention Registration H 1,791 entitled“Stimulated Brillouin Scattering For Fiber-Optic Links” published Mar.2, 1999 and Electronic Letters, vol. 30, no. 23, pp. 1965–1966 (1994) byWilliams and Esner, both of which are hereby incorporated herein byreference.

SBS is a known optical effect. When an optical frequency electromagneticwave causes vibrations (i.e. an acoustic wave) of the density of anoptical medium, an optical grating is produced that causes scattering ofthe electromagnetic wave traveling in the optical medium. In Brillouinscattering, the wavelength of the scattered electromagnetic wave isshifted with respect to that of the original electromagnetic wave due tothe Doppler effect from the motion of the acoustic wave. The frequencyshift is a maximum in the backward direction and it reduces to zero inthe forward direction, which makes SBS a mainly backward directedprocess. The incident optical frequency is also known as the pumpfrequency, which gives the Stokes and anti-Stokes components of thescattered radiation.

In U.S. Statutory Invention Registration H 1,791, the threshold for SBS,which typically depends on the length of the optical fiber and the levelof the optical power input to the fiber, is set to attenuate just theoptical carrier and not attenuate the modulation sidebands or otherfrequency components. The effect of this selective attenuation is toenhance the modulation depth (the ratio of the modulation sideband tothe carrier). The increased modulation depth can improve the performanceof the RF-photonic link. H 1,791 makes use of either a long length ofoptical fiber or a weakly coupled fiber-optic ring resonator as themedium in which the SBS attenuation occurs. The purpose of the ringresonator is to increase the effective length of the SBS medium so thatthe optical path-length is much longer than the physical length of theoptical filter.

In U.S. Pat. No. 6,178,036 to Yao and in IEEE Photonics Letters, vol.10, no. 1, pp. 138–140 (1998), SBS is used to selectively amplify aselected RF-lightwave modulation sideband. An optical pump signal isinjected in the reverse direction into the SBS medium. The optical pumpsignal is offset in frequency by the Stokes shift from the desiredmodulation sideband. Since the frequency of the desired modulationsideband coincides with the frequency of the Brillouin scattering, thesideband is amplified. The purpose of the selective amplification is toselectively amplify the desired modulation sideband and leave the strongcarrier un-amplified. This improves the modulation depth. It alsoproduces a single-sideband modulated signal, which may have benefits ofreduced distortion from optical fiber dispersion. Amplitude modulationof a carrier can produce two modulation sidebands, which have the samemagnitude of frequency offset from the carrier but are offset bypositive and negative values, respectively. In a single-sidebandmodulated signal, one sideband of this pair is substantially strongerthan the other sideband.

The prior art discussed above utilizes the well-known SBS effect toimprove the modulation depth of a RF-modulated lightwave signal or toreduce the distortion from optical fiber dispersion.

The method and system disclosed herein exploits the narrow-band powerlimiting action of SBS to suppress strong interfering signals, whileminimally affecting the desired low-power and/or wide bandwidth receivedsignal.

The relatively narrow bandwidth of gain for SBS in optical fibers isused to produce a peak-power limiter for undesired RF and RF-lightwavesignals. The RF signals are amplitude modulated onto a lightwave carrierto create modulation sidebands. The limiter selectively attenuates thosemodulation sidebands that are stronger than a threshold level.

Thus, only strong frequency components are limited and the weakerfrequency components become enhanced, in comparison. The advantage ofthe SBS approach to limiting is that it is passive, it self-selects thefrequencies attenuated, and it affects only a narrow band at eachattenuation notch. The SBS only affects a narrow band at eachattenuation notch because the spontaneous bandwidth of the SBS effect inan optical fiber is typically smaller than 100 MHz. The system andmethod disclosed makes use of the relatively small bandwidth of the SBSeffect to distinguish between the different modulation sidebands, whichare spaced farther apart than the SBS gain bandwidth. Such frequencyselective limiting is not normally achievable with electronic limiters.In addition, the disclosed system and method seeds the Stokes-shiftedlight into the main SBS medium (in the reverse direction) so that thelength of that main SBS medium can be reduced. This seed is preferablygenerated in a separate ring or strand of fiber from the main SBSmedium.

Previously, as disclosed in U.S. Statutory Invention Registration H1,791, a long length of fiber (generally 25 km or greater) is used tosustain the SBS at common levels of optical power, generally below 10 mWfor the frequency components attenuated. It would be desirable to eitherreduce the amount of optical power required and/or to shorten the lengthof the SBS medium 207 to thereby improve the system's signal-to-noiseratio. The use of shorter fibers in the SBS medium 207 would havereduced passive losses as compared to longer fibers required by theteaching of the prior art. Further, the use of shorter fibers for theSBS medium also has the advantage of reducing the four-wave mixing ofmultiple signal frequencies contained in the signal. Such four-wavemixing generated signals can create spurious noise in a long SBS fibermedium.

SUMMARY OF THE INVENTION

Compared to the prior art, the disclosed system and method taps off aportion of the input signal and diverts it to a separate SBS medium tocreate a seed for SBS that is injected into the main fiber in thereverse direction which allows the length of that SBS medium to bedramatically shortened. In one embodiment of the disclosed system andmethod, the separate SBS medium comprises at least one recirculatingloop and optical amplification is added to the at least onerecirculating loop to further reduce the threshold for limiting.Further, if the frequency of the RF signal to be limited is known, yetanother embodiment of the disclosed system and method modulates thetapped off signal with a RF signal having the frequency to be limitedbefore that RF lightwave signal is supplied to the recirculating loop,which seeds the SBS limiter for that particular sideband frequency.

Additionally, the disclosed system and method utilizes a RF-lightwaveapproach to selectively attenuate certain modulation sidebands and notto attenuate other modulation sidebands.

In one aspect the present invention provides a limiter having atransmitter producing an output signal having at least two frequencycomponents; a signal divider for dividing said output signal into afirst divided signal and a second divided signal; a first SBS medium forreceiving said first divided signal; and a second SBS medium, saidsecond SBS medium generating Stokes light in response to said seconddivided signal, said second SBS medium being coupled to the first SBSmedium for providing said Stokes light thereto.

In another aspect the present invention provides a method forselectively attenuating frequency components, the method including (i)modulating an RF signal onto a lightwave carrier creating a RF-modulatedlightwave signal, the RF-modulated lightwave signal having at least twofrequency components; (ii) dividing the RF-modulated lightwave signalinto a first lightwave signal and a second lightwave signal; (iii)propagating the first lightwave signal into a first SBS medium; (iv)generating a set of Stokes waves from said second lightwave signal; and(v) seeding the first SBS medium with the set of Stokes waves. Thistechnique permits the use of a threshold for the first SBS medium to beset lower than the threshold would have been set without the seed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 a depicts an input spectrum into a limiter;

FIG. 1 b depicts an output spectrum out of a limiter based upon theinput spectrum given in FIG. 1 a;

FIG. 2 depicts the basic elements of a limiter based on a prior artconstruction;

FIG. 3 a depicts a second input spectrum into a limiter;

FIG. 3 b depicts an output spectrum corresponding to the input spectrumof FIG. 3 a;

FIG. 3 c depicts a Stokes reflection from a SBS medium corresponding toan input spectrum shown in FIG. 3 a;

FIG. 4 depicts a limiter including two SBS mediums;

FIG. 5 depicts a limiter where one of the SBS mediums comprises twoloops;

FIG. 6 depicts a limiter including two SBS mediums and a modulator for areference input;

FIG. 7 a depicts a third input spectrum;

FIG. 7 b depicts an output spectrum after the input spectrum of FIG. 7 ahas been sent through the system of FIG. 6;

FIG. 7 c depicts the Stokes reflection from the Main SBS medium afterthe input spectrum of FIG. 7 a has been sent through the system of FIG.6;

FIG. 8 depicts another limiter including two SBS mediums; and

FIGS. 8 a–8 d depict an input spectrum, a carrier-suppressed spectrum,and a monitor-port spectrum, and an output spectrum, respectively, takenat several points in the embodiment of FIG. 8.

DETAILED DESCRIPTION

The limiter system and method disclosed may suppress, in a self-adaptivemanner, much of the unwanted RF interference without needing a prioriknowledge of the frequencies of those interfering signals. This approachmakes beneficial use of Simulated Brillouin Scattering (SBS) in opticalfibers to reduce the power of the undesired signals. The effect of SBSis essentially to limit the output power at all frequencies such thatthe output power at all frequencies falls below a given threshold level101, as illustrated in FIGS. 1 a and 1 b. The spontaneous-gain bandwidthfor Brillouin scattering is approximately 50–100 MHz. One skilled in theart will appreciate this gain bandwidth determines the frequencyresolution of the system and method disclosed herein. Thus, anyinterfering signal that is spaced more than 50 MHz away from the desiredsignal can be reduced without also reducing the intensity of the desiredsignal. The threshold level for SBS depends on the length of the opticalfiber, but is not dependent on the exact frequency, f. Thus, unlike theconventional notch filters used for interference suppression, aSBS-based limiter does not need prior knowledge of the frequencies ofthe interfering signals. Furthermore, as will be further discussedbelow, any known interfering signals can be suppressed to an even lowerlevel by injecting light modulated at the Stokes-shifted replicas ofthose signals in the reverse direction through the fiber.

As discussed above, the disclosed system and method exploits thenarrow-band power limiting action of SBS to suppress strong interferingsignals while not affecting the desired low power and/or wide bandwidthreceived signal. This approach to limiting self-picks the frequenciesattenuated, and affects only a narrow band of frequencies (<50 MHz) ateach attenuation notch. The disclosed system and method reduces thelength of the optical fiber needed to achieve the limiting, therebyreducing the attenuation and distortion of the desired signalcomponents, which do not undergo limiting. Preferred implementations ofthe disclosed system and method for limiting are discussed belowstarting with FIG. 4. Before discussing the preferred embodiments of theinvention, the prior art represented by U.S. Statutory InventionRegistration H 1,791 will first be discussed in greater detail.

FIG. 2 depicts the basic elements of a frequency selective limiter thatis based on a construction disclosed in U.S. Statutory InventionRegistration H 1,791. The attenuator/filter 200 of FIG. 2 has a RF input201 and a RF output 211. The attenuator/filter 200 includes a photonictransmitter 202, an optical isolator 203, a 2×2 coupler 205, a length ofoptical fiber 207 in which SBS occurs (also referred to herein as a SBSmedium) and a photodetector 209. Generally, the RF signal at input 201contains a number of frequency components. The input 201 is applied tothe photonic transmitter 202, which modulates the RF signal at input 201onto a lightwave carrier, thereby creating a RF-lightwave signal. Thelength of optical fiber in the SBS medium 207 performs the limitingfunction. The optical isolator 203 prevents the backward-propagatinglight created by SBS (i.e., the Stokes light) from interfering with theoperation of the photonic transmitter 202. The photodetector 209converts the limited RF-lightwave signal back into a RF signal, which isavailable at the output of the photodetector 211. The 2×2 opticalcoupler 205 is placed between the transmitter 202 and the SBS medium 207to provide a tap for the backward-propagating light. Thus, the frequencycomponents that experience limiting can be monitored and measured usinga second photodetector 213. The output of the second photodetector 213,herein referred to as monitor port 215, provides a means to determinethe frequencies and strengths of the interfering signals.

Stimulated Brillouin scattering (SBS) has been studied extensively since1964 and in fibers especially since mid-1970s. For more information onSBS the reader may wish to review the book Nonlinear Fiber Optics,Academic Press, 1995, by G. P. Agrawal. SBS manifests itself throughcoupling the energy of a “pump” beam to a backward propagating “Stokes”beam which is down shifted in frequency with respect to the pump beam byν_(B)=2ν_(s)/ (c/n)ν_(p)(ν_(B) is about 10.5 GHz for 1.55 μm light),where ν_(s), and (c/n) are the speeds of the sound and light in thefiber, and ν_(p) is the optical frequency of the pump beam. For smallsignals, the growth of the Stokes beam can be described by anexponential relation, exp[g(ν) (P_(p)/A_(c))L_(eff)]. Here g(ν) is thegain coefficient, P_(P) is the pump power, A_(c) is the effective corearea of the fiber and L_(eff) is the effective fiber length. L_(eff)equals [1−exp(−αL)]/α, where L is the fiber length and α is theabsorption coefficient. Generally, g(ν) has a Lorentzian line shape witha peak at ν_(s)=ν_(p)−ν_(B), and bandwidth Δν_(B). Typically, Δν_(B) isabout 30–50 MHz for silica fibers at 1.55 μm but it can be broader ifthere are inhomogeneities in the fiber due to manufacturing, temperatureor stress variations along the fiber. Since the intrinsic SBS linewidthin the fiber, Δν_(B), is <50 MHz, SBS is most easily observed withnarrowband pumps. Typically, g(ν_(s)) has a value of about 5×10⁻¹¹ cm/Wfor pure silica fibers and it is wavelength independent. Given thesetypical values in the fiber, spontaneous SBS has been observed with only5 mW of input power, P_(thres), at 1320 nm wavelength in 13.6 km longfibers, when g₀L_(eff)(=g(ν) (P_(thres)/A_(c))L_(eff)) reaches a valueof 15–30 as described in a paper by D. Cotter, Electronics Letters,1982, vol. 17, p. 379. For input powers higher than the SBS thresholdpump power P_(thres) the transmitted power is clamped to approximatelyone or two times (P_(thres)) exp(−αL) and the excess power,(P_(P)−P_(thres)), is converted into a strong Stokes beam propagating inthe backward direction. The effective bandwidth of the interaction alsois reduced, to Δν_(B)/(g₀L_(eff))^(1/2), due to exponential gain in thestimulated scattering process.

FIG. 3 a depicts the spectrum at the input to the SBS medium 207. InU.S. Statutory Invention Registration No. H 1,791, the threshold for theattenuator/filter 200 of FIG. 2 is typically set to attenuate theoptical carrier 303 of FIG. 3 a, and thereby increases the depth ofoptical modulation of the RF-lightwave signal. The optical carrier 303typically has much higher optical power than the modulation sidebands307 a–d. This setting is indicated by the upper dashed line 301 of FIG.3 a.

The prior art attenuator/filter 200 of FIG. 2 can be used in a differentmanner than that described in U.S. Statutory Invention Registration No.H 1,791. The attenuator/filter 200 can be used to selectively limit theintensities of strong RF components 305 a–f, i.e. the undesiredinterference signals, which can be observed in the spectrum of FIG. 3 aas features or peaks in the modulation sidebands. The stronginterference peaks 305 a–f can be selectively limited by setting thethreshold for SBS just above the intensities of the desired signal peaks307 b, 307 c, so that those desired features are affected only minimallyby SBS. This may be accomplished by increasing the length of the opticalfiber in the SBS medium 207 or by increasing the optical power of thetransmitter 202. The new setting is indicated by the lower dashed line311 in FIG. 3 a. FIG. 3 b illustrates the output from the SBS medium 207where all of the signals have been limited to the level indicated byline 311. FIG. 3 c illustrates the Stokes light observed from the 2×2coupler 205 tap when the threshold of the attenuator/filter 200 is setto the lower dashed line 311 of FIG. 3 a.

This approach has several weaknesses, however. First, the opticalcarrier is attenuated by the SBS process. Since SBS is derived fromnoise, the attenuated optical carrier is more noisy. This noise istransferred to the desired RF signal components produced byphotodetector 209. Second, a long length of the fiber 207 or highoptical intensity from the transmitter 202 is needed for selectivelimiting to occur.

FIG. 4 illustrates an embodiment of a self-adapting limiter inaccordance with the present invention. The signal output from a photonictransmitter 402 is a RF modulated lightwave signal that contains anumber of modulation sidebands as well as the optical carrier. Thissignal is passed through an isolator 403, a splitter 423, and anoptional optical circulator 424 and then into a SBS medium 407 whereinSBS limits the transmitted power at the frequencies of the strong,co-site interference ν_(I) or jamming ν_(J) signals. Only the lightpower in a narrow band (<Δν_(B)) of frequencies around ν_(p)=ν_(O),ν_(I), ν_(J) are affected. SBS thresholds at a 1550 nm wavelength havebeen measured for several common types of optical fiber as discussed ina paper by C. C. Lee and S. Chi, IEEE Photonics Technology Letters,2000, vol. 12, no. 6, p. 672. Thresholds for spontaneous (unseeded) SBSare between 5–10 mW for 25 km lengths of fiber. A coupler may be used inlieu of the splitter 423 and circulator 424.

As shown in FIG. 4, in addition to the SBS medium 407, a second SBSmedium 429 is utilized in this embodiment. In this embodiment, thesecond SBS medium 429 takes the form of an SBS ring. One skilled in theart will appreciate that there are other SBS media known in the art thatcan be used and that SBS medium may comprise one or more loops or fiberstrands, as will be discussed later in reference to FIGS. 5 and 8. Asthe RF-modulated lightwave 401 is passed through splitter 423, a portionof the RF-modulated lightwave is directed toward the SBS ring 429, whilethe remaining portion of the RF-modulated lightwave continues toward theSBS medium 407. A portion of the RF-modulated lightwave passes throughan optional optical amplifier 425 and is then coupled into the SBS ring429 by an input coupler 427. The RF-lightwave circulates in the SBS ring429 and Stokes light is generated. The generated Stokes light leaves theSBS ring 429 through an output coupler 431. The Stokes light from ring429 then passes through a desirable optical notch filter 433 and anoptional optical amplifier 435 before entering into a second opticalcirculator 437, where the Stokes light from ring 429 enters the SBSmedium 407.

The addition of the SBS ring 429 allows the power of transmitter 402 tobe reduced and/or allows the fiber length of the SBS medium 407 to bereduced. With this approach, a fraction of the input signal 401(typically, 0.1–1.0 mW) is fed into the SBS ring 429 using splitter 423.The SBS ring 429 may contain a single loop as depicted in FIG. 4. TheSBS ring 429 recirculates both the input light from the splitter 423 andthe generated Stokes shifted light, at frequency ν_(s), produced by SBSin the SBS ring 429. The threshold pump power for SBS oscillations isreduced in the SBS ring 429 because of this recirculation. The thresholdin the SBS ring 429 is preferably set so that only the componentsν_(p)=ν_(O), ν_(I), ν_(J) produce appreciable Stokes shifted energy. Aportion of the Stokes shifted light from the ring is tapped off by theoutput coupler 431 and is fed into the distal end of the main SBS medium407 such that it counter-propagates to the signal carrying light. In theSBS fiber medium 407, the Brillouin amplification of this light at ν_(s)depletes only the components at ν_(p). This results in a Stokes seed forthe SBS effect in the SBS medium 407. For example, with a seed energy of0.1–1.0 mW, a much smaller SBS gain, g₀L_(eff)<<10, is sufficient toreach a depletion regime of operation. Thus, a more practical length(˜100 to 1000 m) of SBS fiber may be used in the SBS medium 407 (asopposed to the 25 km lengths required by the prior art). A smallerlength of SBS fiber provides the benefits of reducing the attenuation,from propagation loss, of the desired signal components and reducingspurious noise that can be generated by four-wave mixing of differentsignal frequencies.

A pair of optical couplers 427, 431 is used to couple light into and outof the SBS ring 429. For a high-Q SBS ring resonator 429, each ofsidebands associated with the interfering signal components shouldcoincide with one of the resonance frequencies of the SBS ring 429.Since the interfering signals are generally unknown, this is generallynot feasible. Thus, the SBS ring resonator 429 is designed to have a lowexternal (loaded) Q. Therefore, the strengths of the input and outputcouplers 427, 431 are fairly high. One skilled in the art willappreciate that the external Q is determined primarily by the couplingstrengths of the input and output couplers 427, 431 since the fiberattenuation is so small. The light makes only a few cycles through theSBS ring resonator 429. Thus, the SBS ring resonator 429 must be quitelong in order for the SBS threshold to be sufficiently low. Therefore,with the addition of the SBS ring 429, the length of the main SBS medium407 (which affects the signal-to-noise property of the desired signals)is reduced at the expense of adding a long length of fiber in the SBSring 429 (which affects only the undesired interference).

The optional optical amplifiers 425, 435, can be located between thetransmitter 402 and the SBS ring 429 or between the SBS ring 429 and thecirculator 437 that feeds the Stokes seed into the main SBS medium 407.These amplifiers 425, 435, if utilized, further increase the strength ofthe Stokes seed and further reduce the SBS threshold level or the lengthneeded of the main SBS medium 407. These amplifiers 425, 435 willincrease the noise of the Stokes seed, and thus increase the noise ofthe undesired interference components, but they do not affect the noiseof the desired signal components.

One or more notch filters 435 may be inserted between the output coupler431 and the optical circulator 437, so that essentially no Stokes seedis generated for those desired frequency components. The optional notchfilter 435 may be designed to remove the Stokes light associated withany one of the frequency components that is generating Stokes light. Atleast one notch filter 435 is preferably utilized to remove the Stokeslight associated with the optical carrier. The removal of the Stokeslight associated with the optical carrier prevents the optical carrierfrom being reduced. One skilled in the art will appreciate that the SBSprocess can add noise to those signal components that are attenuated byit. Thus, it is desirable to avoid having the SBS process affect theoptical carrier, since added noise from the SBS process can otherwisedegrade the signal to noise ratio of the desired RF signal componentsproduced by photodetector 409.

FIG. 5 depicts another embodiment in accordance with the presentinvention. Elements previously described in relation to FIG. 4 that arethe same as those in FIG. 5 are labeled with the same numerical numbersand function essentially as previously described. In this embodiment, anamplifier 450 is preferably included in the SBS ring 429. Amplifier 450amplifies both the tapped signal light and the Stokes shifted energy,produced from the components of the input signal at ν_(p)=ν_(O), ν_(I),ν_(J). The intended effect of amplifier 450 is to further increase thestrength of the Stokes seed and further reduce the SBS threshold levelor the length needed of the main SBS medium 407. Amplifier 450 willincrease the noise of the Stokes seed, and thus increase the noise ofthe undesired interference components, but the amplifier 450 does notaffect the noise of the desired signal components.

The embodiment of FIG. 5 illustrates several other possible embodimentsof the invention. For example, the output coupler 431 of FIG. 4 may bereplaced by a splitter 431 a of FIG. 5. Light is coupled out of the ring429 through the splitter 431 a essentially regardless of its frequency.

Another possible modification illustrated in FIG. 5 is the addition of asecond ring 452 coupled to the SBS ring 429 by a 2×2 coupler 454. Thismodification results in a SBS dual-loop ring resonator comprising twocoupled loops of optical fiber 452, 429 that trace a figure-eight path.The SBS dual-loop ring resonator provides enhanced functionality overthe single loop SBS ring 429 of FIG. 4. In some applications, theinterference components cover a small range of frequencies(substantially less than 10 GHz). It may be desirable to tune thecoupled loops 429, 452 to achieve a resonator resonance spectrum thatcontains peaks primarily at frequencies of the interference componentsand their Stokes-shifted light. An optional phase shifter 456 in thefirst loop 452 ensures that the light in the first loop 452 is in phasewith the light from the transmitter 402 that is injected into thedual-loop ring resonator at the splitter 423. The optional phase shifter456 accommodates constructions tolerances and thus the added phase shiftis adjusted as needed.

FIG. 6 illustrates another embodiment of the invention. In thisembodiment a modulator 465 has been added to the system between thesplitter 423 and the coupler 427 for SBS ring 429. If the frequency,ν_(K), of an interfering signal is known a priori, a reference signal463 at that frequency can be injected into the SBS ring resonator bymeans of the modulator 465. The reference signal 463 is modulated ontothe RF-lightwave signal diverted into the SBS ring 429. The modulator465 is placed before the SBS ring 429 and adds ν_(K) to the frequenciesthat produce the Stokes seed. Thus, the component of the input signal atν_(K) also will be suppressed by the SBS in the SBS medium 407. Oneskilled in the art will appreciate that the frequency of the referencesignal 463 does not have to be exactly the same as the frequency of theinterfering signal to be attenuated. The frequency of the referencesignal 463 only has to be separated from the interfering signal by anamount substantially less than the SBS gain bandwidth.

FIG. 7 a depicts the input spectrum at the input to the transmitter 402along with a line 701 indicating where the SBS threshold is set for thelimiter of FIG. 6. The spectrum includes known strong interferingsignals 704 a-1, 704 b-1, known weak interfering signals 707 a-1, 707b-1 and desired signals 702 a, 702 b. As discussed above, the referencesignal 463 is supplied separately from the RF input signal 401,therefore, the frequency component of the known strong interferingsignals 704 a-1, 704 b-1 and the known weak interfering signals 707 a-1,707 b-1 can be made even weaker, at the RF output 411, than the levelset by the SBS threshold 701. FIG. 7 b depicts the output spectrum atthe RF output 411. As illustrated in FIG. 7 b, the frequency componentat the RF output 411 of the known strong interfering signals indicatedby reference 704 a-2, 704 b-2 and the frequency component of the knownweak interfering signals indicated by reference 707 a-2, 707 b-2 areattenuated such that they are below reference line 701 and desiredsignals 702 a, 702 b.

FIG. 8 depicts another embodiment including two SBS mediums. In thisembodiment, the second SBS medium 429 comprises a strand of fiber theremote end of which has a non-reflective termination that may beutilized for monitoring purposes, if desired. Since the second SBSmedium has only one end (a proximate end) that is coupled to thecircuit, a single circulator (labeled 427′, 431′) replaces couplers 427,431 and splitter 431 a described with reference to the priorembodiments. In this embodiment, the notch filter 433 is moved closer tosplitter 423 and in front of the second SBS medium 429 making theoptical amplifier 425 more efficient since the optical carrier is notalso being amplified in this embodiment. The isolator 403 is moveddownstream of the splitter. An optional polarization rotator may beutilized if the other components coupled directly or indirectly to thesecond SBS media are not polarization maintaining. Otherwise the opticalcircuit of FIG. 8 is similar to the previously discussed embodiments andsince common reference numerals are used to identify the same orsimilarly functionally components in the several embodiments, theremaining components shown in FIG. 8 should not require furtherexplanation for those skilled in the art in view of the priorexplanations which have been provided.

The main SBS medium 407 is not expected to have much effect on thedesired frequency components of the input signal, if those componentsare separated from the filtered components by more than the SBSbandwidth. The effect on the desired signal components is similar to theeffect of propagating a weak optical signal in a long length of fiber.Thus, the SBS limiters of FIGS. 4–8 respectively are expected to havehigh linearity. However, one must consider the interaction between thevarious strong, unwanted frequency components, that may arise because ofother non-linear effects (e.g., four-wave mixing) occurring in theoptical fiber. The potential exists for that interaction to create spurswithin the bandwidths of the desired signal components. One skilled inthe art will appreciate that the Stokes signal is strongest in theportion of the main SBS medium 407 that is closest to the photonictransmitter 402. Thus, the unwanted components are attenuated quickly bySBS and their optical intensity is close to the limited value over muchof the length of the SBS medium 407. This effect should reduce thecontribution from these other fiber non-linearities.

Having described the invention in connection with a preferred embodimenttherefore, modification will now certainly suggest itself to thoseskilled in the art. As such, the invention is not to be limited to thedisclosed embodiments except as required by the appended claims.

1. A limiter comprising: a transmitter producing an output signal havingat least two frequency components; a signal divider for dividing saidoutput signal into a first divided signal and a second divided signal; afirst stimulated Brillouin scattering medium for receiving said firstdivided signal; and a second stimulated Brillouin scattering medium,said second stimulated Brillouin scattering medium generating Stokeslight in response to said second divided signal, said second stimulatedBrillouin scattering medium being coupled to the first stimulatedBrillouin scattering medium for providing said Stokes light thereto. 2.The limiter of claim 1 wherein the stokes light coupled to the firststimulated Brillouin scattering medium propagates in a reverse directioncompared to said first divided signal received by the first stimulatedBrillouin scattering medium.
 3. The limiter of claim 2 furthercomprising an optical notch filter, wherein said Stokes light generatedby said second stimulated Brillouin scattering medium passes throughsaid optical notch filter before being provided to said first stimulatedBrillouin scattering medium, said Stokes light comprises a Stokes wavecorresponding to an at least one of the at least two frequencycomponents, said optical notch filter removing said Stokes wavecorresponding to said at least one of the at least two frequencycomponents before passing said Stokes light from said second stimulatedBrillouin scattering medium to said first stimulated Brillouinscattering medium.
 4. The limiter of claim 3 wherein the at least one ofthe at least two frequency components comprises Stokes light associatedwith an optical carrier of the transmitter.
 5. The limiter of claim 4wherein the optical notch filter block Stokes light associated with saidoptical carrier.
 6. The limiter of claim 1 further comprising a firstoptical amplifier for amplifying said second divided signal.
 7. Thelimiter of claim 6 further comprising a second optical amplifier foramplifying said Stokes light from said second stimulated Brillouinscattering medium.
 8. The limiter of claim 7 further comprising a thirdoptical amplifier for amplifying said Stokes light within said secondstimulated Brillouin scattering medium.
 9. The limiter of claim 1wherein said second stimulated Brillouin scattering medium comprises oneor more optical loops.
 10. The limiter of claim 1 wherein said secondstimulated Brillouin scattering medium comprises a strand of opticalfiber having a non-reflective distal end, said distal end beingoptically isolated from other elements of said limiter.
 11. The limiterof claim 1 further comprising a modulator for modulating said seconddivided signal with a reference input before said second stimulatedBrillouin scattering medium receives said second divided signal.
 12. Thelimiter of claim 11 wherein said reference input is associated with atleast one of the at least two frequency components.
 13. The limiter ofclaim 1 further including an optical notch filter and wherein the seconddivided signal passes through said optical notch filter before beingcoupled to said second stimulated Brillouin scattering medium.
 14. Thelimiter of claim 13 wherein the at least one of the at least twofrequency components comprises Stokes light associated with an opticalcarrier of the transmitter.
 15. The limiter of claim 14 wherein theoptical notch filter block Stokes light associated with said opticalcarrier.
 16. A method for selectively attenuating frequency componentscomprising the steps of: modulating an RF signal onto a lightwavecarrier creating a RF-modulated lightwave signal, said RF-modulatedlightwave signal having at least two frequency components; dividing saidRF-modulated lightwave signal into a first lightwave signal and a secondlightwave signal; propagating said first lightwave signal into a firststimulated Brillouin scattering medium; generating a set of Stokes wavesfrom said second lightwave signal; and seeding said first stimulatedBrillouin scattering medium with said set of Stokes waves, whereby athreshold for said stimulated Brillouin scattering medium may be setlower than the threshold would be set without said seed.
 17. The methodof claim 16 further comprising removing at least one frequency componentfrom said second lightwave signal before said set of Stokes waves isgenerated from said second lightwave signal.
 18. The method of claim 17wherein said at least one frequency component comprises an opticalcarrier.
 19. The method of claim 16 wherein said set of Stokes wavescontains a first Stokes wave corresponding to at least one of said atleast two frequency components, said method further comprising the stepof removing said first Stokes wave from said set of Stokes waves beforeseeding said first stimulated Brillouin scattering medium.
 20. Themethod of claim 19 wherein at least one of said at least two frequencycomponents comprises an optical carrier and wherein the first Stokeswave is removed before seeding said first stimulated Brillouinscattering medium.
 21. The method of claim 16 further comprising thestep of amplifying said second lightwave signal.
 22. The method of claim16 further comprising the step of amplifying said set of Stokes waves.23. The method of claim 16 wherein the step of generating the set ofStokes waves further comprises recirculating said second lightwavesignal in one or more loops of a SBS medium.
 24. The method of claim 16wherein the step of generating the set of Stokes waves further comprisespropagating said second lightwave signal in a strand of optical fiberhaving a non-reflective distal end, said distal end being opticallyisolated from other elements of said limiter.
 25. The method of claim 16further comprising the step of modulating said second divided signalwith a reference input before generating said set of Stokes waves. 26.The method of claim 25 wherein said reference input is associated withat least one of the at least two frequency components.
 27. A filter forselectively attenuating frequency components of a signal, the filtercomprising: a modulator for modulating the signal onto a lightwavecarrier creating a RF-modulated lightwave signal, said RF-modulatedlightwave signal having at least two frequency components; a splitterdevice for dividing said RF-modulated lightwave signal into a firstlightwave signal and a second lightwave signal; a first stimulatedBrillouin scattering medium coupled to receive the first lightwavesignal; means for generating a set of Stokes waves from said secondlightwave signal; and means for seeding the first stimulated Brillouinscattering medium with the set of Stokes waves.